Induction motors are single-phase motors with a series excitation commutator and are frequently used in the field of domestic electrical appliances, for example for driving the rotating drum of a clothes washing or drying machine. Such motors are often powered by a semiconductor triac device. It is often desired to control the speed of such a motor to increase the efficiency of its operation.
Known methods for controlling the speed of rotation of a universal motor use a tachometric generator coupled to the motor to supply an output electrical signal that is representative of the motor speed and a triac, whose triggering angle determines the RMS (root mean square) voltage applied to the motor windings. Digital control methods are used to control the motor speed based on the principal that a triggering angle difference of the triac is proportional to the motor speed difference.
One such control method is disclosed in U.S. Pat. No. 6,633,149 in which a method for digital control of a universal motor, in particular for electrical household appliances, measures the motor rotation speed, determines the difference between the measured speed and a preset (desired) speed, and controls the motor on the basis of this difference. This method further estimates at least one of the values of the resisting torque of the motor and the current in the motor windings. While such a method is useful, it is relatively complicated to implement particularly in household appliances, such as a laundry machine, where reduced cost is a paramount goal. Accordingly, a need exists for a method of controlling the speed of a motor powered by a triac that is easy to perform and relatively inexpensive to implement.